As one of automatic transmissions, a continuously variable transmission is known. This mechanism includes an input shaft sheave and an output shaft sheave. The input shaft sheave is provided on the input shaft to which the power from an internal combustion engine is input. The output shaft sheave is provided on the output shaft that outputs the power input to the input shaft to the drive wheel. Each sheave includes a movable sheave element and a fixed sheave element. The movable sheave element has a piston and the location of the piston may be varied along the axial direction by hydraulic pressure occurred in a hydraulic pressure chamber. The fixed sheave element is coupled to the input shaft or the output shaft in such a manner that the location of the fixed sheave element along the axial direction thereof is fixed. A groove having a generally V-shaped cross section is formed between the two sheave elements and a V-belt is wound around the input and the output shaft sheaves in the grooves. In such a continuously variable transmission, the width of the grooves in which the V-belt is wound around, or the sheave width, are continuously changed. Accordingly, the stepless change in the transmission gear ratio is achieved.
Generally, a bearing is used to reduce the friction between a rotary shaft which the bearing supports and attachment portion of the rotary shaft. A radial bearing is a bearing that deals with a radial load which acts in a direction perpendicular to the rotary shaft. Each of the input shaft and the output shaft of the above-described continuously variable transmission is rotatably supported at one end by the radial bearing that is fit over the shaft connected to the movable sheave element and the fixed sheave element.
When the radial bearing supports the rotary shaft, it is possible that the bearing drops off from the rotary shaft by the action of the thrust load generated from hydraulic pressure in the hydraulic pressure chamber. To suppress this, for example, Japanese Laid-open Patent Publication 2000-220710 discloses a nut that is threaded on the end of the rotary shaft. More specifically, referring to FIG. 1A, a radial bearing 33 includes an inner race 33a, an outer race 33b, and a ball 33c between them. The inner race 33a is fit over one end of a shaft 32 that is connected to a fixed sheave element. A threaded portion 32d is formed on the shaft 32 at a position closer toward the end than the position where the inner race 33a is press-fitted. A threaded portion 34d of the nut 34 mates with the threaded portion 32d of the shaft 32.
Japanese Laid-open Patent Publication 2006-132703 discloses a bearing structure in which a nut for fixing a bearing is omitted, as illustrated in FIG. 1B. As illustrated, threaded portion 43d is formed on the inner periphery of an inner race 43a of a radial bearing 43 and a threaded portion 42d is formed on the outer periphery of a shaft 42 connected to a fixed sheave element. The threaded portions 43d and 42d mates together.
In the above bearing structures of JP-A-2000-220710 and JP-A-2006-132703, the cross-sectional shape of the threaded grooves of the threaded portions 32d and 42d is a notch. For this reason, stress concentration generates at the bottom of the threaded groove. Generation of the stress concentration may causes cracks at the bottom thereby deteriorating fatigue strength of the shafts 32 and 42.
This problem is not limited to a bearing structure of the continuously variable transmission but common to bearing structures including a radial bearing for rotatably supporting an end of the rotary shaft while the radial bearing receives a thrust load.